As a material for optical lenses for use in optical systems of various cameras such as a camera with a film integrated and a video camera, an optical glass or an optical resin is used. The optical glass is excellent in heat resistance, transparency, dimensional stability and chemical resistance, while it has problems of a high material cost, poor moldability and low productivity.
An optical lens formed of the optical resin has an advantage that can be mass-produced by injection molding. For camera lenses, a polycarbonate resin, etc., are used. In recent years, however, there is demand for developing high-refractivity resins, owing to requirements for products to be lighter in weight and more compact in size. Generally, when an optical material has a high refractive index, a lense element having the same refractive index can be materialized with a surface having a smaller curvature, so that the amount of aberration which occurs on the surface can be decreased, that the number of lenses can be decreased, that the eccentric sensitivity can be decreased and that the thickness of a lens can be decreased to render the element lighter.
Further, when the optical resin is used for an optical lens, it is required to have heat resistance, transparency, low water absorption, chemical resistance, light resistance, low birefringence and wet heat resistance in addition to refractivity and Abbe's number. The consequent disadvantage is that the place of use is limited depending upon a balance among properties of a resin. In particular, with an increase in resolution owing to an improvement in the number of pixels in recent years, there is demanded a lens having high imaging performances and lower birefringence. As a method for decreasing the birefringence, generally, there is employed a method in which offsetting birefringences by using a material having positive birefringence and a material having negative birefringence. Therefore, an amount ratio of the material having positive birefringence to material having negative birefringence is very important. For taking a picture of a clear image, the lens is required to maintain high transmittance in all the wavelengths of the visible light region.
There has been therefore developed those resins for an optical lens which have high refractivity, low birefringence and an excellent balance among physical properties. For example, there has been proposed a polyester comprising a fluorene-containing dihydroxy compound and naphthalenedicarboxylic acid (Patent Document 1). However, although the above polyester has high refractivity, it has high birefringence since the positive and negative birefringences are not offset each other. Further, since it has a large content of naphthalene, an absorption derived from a naphthalene ring appears at 380 to 400 nm, and the transmittance thereof to visible light is sometimes greatly decreased.
There is also proposed a polyester carbonate comprising 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene and an aromatic dicarboxylic acid (Patent Documents 2 and 3). However, the above proposal fails to define the aromatic dicarboxylic acid in range or type, and the refractive index is insufficient depending upon aromatic dicarboxylic acids used. Further, since nothing has been studied with regard to the birefringence and transmittance to visible light, the birefringence becomes large depending upon a compositional ratio of the 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene to aromatic dicarboxylic acid, and the transmittance to visible light is sometimes decreased.
Further, optical lenses are used for various purposes, while they are different in form and size depending upon such purposes. For example, the camera lens for a cellphone is very compact and thin like φ5 mm and a thickness of about 0.3 mm, and it is getting more compact and thinner in recent years. When such a compact and thin lens is produced by injection molding, if the flowability of a resin is insufficient, there are some cases where lenses having an intended form cannot be obtained. Since the flowability of a resin is determined by its molecular weight, molecular structure and copolymer composition, the flowability of a resin having such a molecular weight and a copolymer composition as those in Patent Documents 2 and 3 is insufficient, and it is difficult to use such a resin for a compact and thin optical lens.
Further, when a polyester carbonate is produced, it is very important to control the reduced pressure degree. When a polyester carbonate is produced, two monohydroxy compounds having different boiling points are by-produced unlike a polyester or polycarbonate, so that it is very important to control the speed of distilling the by-products off on the basis of the reduced pressure degree during the production. The production method described in Patent Documents 2 and 3 carry out no pressure reduction at an initial stage of a reaction, so that a phenol is not distilled off, and there is a problem that the reaction time period is very long.    (Patent Document 1) JP2006-335974A    (Patent Document 2) JP10-87800A    (Patent Document 3) JP2002-309015A